The present invention generally relates to security paper and, in particular, to security paper having security threads, and to processes for making the security paper.
Many documents of value, such as bank notes, currency, checks, stock certificates, and bonds, are provided with security features for preventing illicit copying and forgery. One such security feature is the use of security paper which is not widely available and difficult to simulate. There are a number of known security features that may be included in security paper, one of which is the inclusion of threads of various materials in the paper.
Such security threads may typically consist of metallic threads, colored threads, optical threads or magnetic threads. Embedded metallic threads beneath the surface of the security paper are not readily apparent in reflected light but are immediately apparent in transmitted light, as a dark image of the threads is seen when the document is illuminated from behind. Metallic threads are simple in concept, but provide an effective anti-copying function. The optical feature of metallic threads cannot be copied by a photocopier or simulated by printing a line on the surface of the security paper.
Colored threads consist of thin filaments of colored material which are typically below, but sufficiently close to, the surface of the paper so as to be visible. Colored threads in security documents are typically apparent upon visual inspection under normal lighting, and are common in currency. Optical threads are filaments of material which are reflective, diffractive or fluorescent. Such threads are sufficiently close to the surface of the paper as to be readily identifiable in ambient light or ultraviolet light. Magnetic threads are filaments of material that are typically identifiable by machine. Such threads may be formed of any one of a number of magnetic materials. Security threads of any of these types may be formed into readily identifiable characters for further visual confirmation of document authenticity.
Security threads are typically embedded into the security paper at the time that the paper is manufactured. Such a papermaking process is very expensive, adding significantly to the overall cost of the paper. Accordingly, there remains a need in the art for security features which may be added to less expensive paper, after the paper is manufactured (i.e., post-manufactured), thereby reducing the manufacturing costs of the security paper.